The present invention relates to a method and system in an automotive vehicle for automatically detecting a preceding vehicle which ensures continuous detection of the presence or absence of the preceding vehicle travelling in the same lane as the vehicle. The system and method according to the present invention is applicable to systems which automatically track a preceding vehicle at a safe intervehicle distance.
Recently, automatic cruising speed control systems have been installed on automotive vehicles for the purpose of maintaining the vehicle approximately at a set cruising speed without manual intervention. Furthermore, other automatic speed control system have been proposed which automatically track the preceding vehicle. This kind of vehicle speed control system controls the opening angle of the throttle valve installed within a throttle chamber in the engine of the vehicle so that the vehicle is maintained at an approximately constant speed or so that the intervehicle distance between the vehicle and a preceding vehicle is held approximately at a predetermined distance. This kind of vehicle speed control system is exemplified by laid-open, unexamined Japanese Patent Application No. Sho 55-86000.
Such vehicle speed control systems require accurate measurement of intervehicle distance from the vehicle to the preceding vehicle in order to carry out appropriate control of the vehicle speed. Therefore, systems have been proposed with radar units mounted on the vehicle in order to determine the intervehicle distance. Such radar units normally employ electromagnetic waves with high directivities, such as radio waves and lasers, as measurement media for accurate measurement of the intervehicle distances, assuming that the transmitted electromagnetic waves are reflected by the preceding vehicle. However, measurement of the intervehicle distance often is impossible, especially in cases where the vehicle is travelling on a curved road since such radar units cannot receive waves reflected by the preceding vehicle.
In addition, in this case, such radar units may erroneously detect forwarding vehicles travelling in adjacent lanes.
Another vehicle speed control system which ensures detection of the presence or absence of a preceding vehicle on a curved road and enables continuous measurement of the intervehicle distance has been proposed. The proposed vehicle speed control system is disclosed in a laid-open, examined Japanese Patent Application No. Sho 51-7892.
FIG. 1 conceptually shows the construction of the above-described vehicle speed control system which detects obstacles in front of the vehicle as disclosed in the above-identified Japanese documents.
In FIG. 1, a rotating unit 9 for rotating the radar unit 1 about a vertical axis is provided so that the radar unit 1 can pivot left and right according to the angular postion of the vehicular steering wheel monitored by a steering angle sensor 5. That is to say, a signal processing circuit 7 in the vehicle speed control system shown in FIG. 1 ensures that the radar unit will always be pointing in the current direction of travel of the vehicle. This helps track any vehicles preceding the vehicle in the lane so as to facilitate measurement of the intervehicle distance, and control of the vehicle speed via a speed adjusting mechanism 11 independent of an usual accelerator pedal so as to maintain the intervehicle distance at a safe intervehicle distance on the basis of the measured distance and the vehicle speed detected by means of a vehicle speed sensor 3.
However, even this method of rotating the radar unit according to a steering angular position cannot fully ensure accurate tracking and measurement since the angular displacement of the steering wheel does not always coincide with the radius of curvature of the curved road. Consequently, a highly reliable detection of the presence or absence of the preceding vehicle cannot always be achieved.
The copending U.S. patent application by the same Applicant, namely, Ser. No. 739,152 filed on May 30, 1985 discloses a similar preceding vehicle detection system in which the steering angle sensor is provided to determine whether the current angular position of the steering wheel is within overlapping areas of any one of the plurality of angles of coverages of the individual laser beams, thus determining whether the preceding vehicle to be tracked by the vehicle moves on the straight road or curved road.